The present invention relates to golf ball manufacturing, and more particularly, to methods and systems of golf ball manufacturing that precisely control the retraction of mold pins based on feedback sensor data, to avoid the formation of imperfections in finished golf balls.
Conventional golf ball manufacturing techniques typically form layers of a golf ball by molding material around a sub-part, such as a solid core or a multilayered preformed golf ball portion. The molding material is typically a thermoplastic or thermoset material that is injection molded around the sub-part. The conventional injection molding techniques customarily use a two-part mold, with each part of the mold defining a hemispherical cavity. The two mold parts mate together with their cavities aligned to define the shape and size of the layer being formed over the sub-part. When the two mold parts are brought together, and before the molding material is injected, the sub-part of the golf ball is usually supported centrally within the cavities by retractable mold pins, leaving a mold cavity of uniform thickness in which to mold the outer layer over the sub-part. The mold pins typically have a pre-load or “pinch” against the sub-part to hold the sub-part in place. Molten molding material is then injected into the mold cavity through several gates. The molding material flowing from each of the gates surrounds the sub-part, converges, and fills the mold cavity between the golf ball sub-part and the mold. In filling the mold cavity, the molding material impinges on and surrounds the mold pins. Once the mold cavity is filled, but before the outer layer has completely hardened, the mold pins holding the sub-part retract so that the molding material may fill the voids left by the pins. The molding material then cools and hardens to form the outer layer, which may be an intermediate layer of the golf ball (e.g., an inner layer or mantle layer) or the cover of the golf ball.
In these conventional golf ball injection molding systems, the initially injected molten molding material fills the mold cavity and may contact and surround the mold pins before the mold pins are retracted. The pins must remain extended a duration sufficiently long enough to prevent movement of the sub-part during injection. In preventing this movement, however, when the pins are eventually retracted, the pins may leave gaps within the initially injected molten molding material. If the pins are held against the sub-part too long, the delay in filling the gaps with additional molten molding material may create a difference in cooling and hardening between the material in the gaps and the remaining material filling the mold cavity. Consequently, imperfections can form at or around the gaps left by the mold pins. For example, the retracting pins can leave cavities beneath the outer surface of the outer layer. In addition, fine cracks, “crows foot” marks, weld marks, and other boundary imperfections may be caused by the molten material flowing around the mold pins and forming knit lines or weld lines as the additional material fills the gap (e.g., circular space) vacated by a mold pin. Detrimentally, the imperfections around the mold pins may create weak spots on the finished ball and lead to premature cracking of the ball.